Understanding Malathion and Its Primary Use
Malathion is an organophosphate insecticide that serves as a powerful prescription treatment for ectoparasites [1.5.2, 1.2.1]. Its primary and FDA-approved application is the topical treatment of head lice (Pediculus humanus capitis) infestations [1.2.2]. It is available as a 0.5% lotion and is effective against both live adult lice and their eggs, often called nits [1.2.1, 1.5.2]. This dual action makes it a valuable tool, especially when over-the-counter (OTC) treatments fail or when lice show resistance to other common insecticides like permethrin [1.7.4]. Malathion is typically prescribed for adults and children 6 years of age and older [1.2.1]. Its use is not recommended for infants due to higher skin permeability and potential for increased absorption [1.12.1].
How Malathion Works
The mechanism of action for malathion is neurological. It is an acetylcholinesterase inhibitor [1.5.4]. Inside the louse, malathion is converted into its more potent metabolite, malaoxon [1.5.2]. This substance irreversibly binds to and deactivates the acetylcholinesterase enzyme at the nerve synapses of the insect [1.5.4]. Normally, this enzyme breaks down the neurotransmitter acetylcholine, ending nerve signals. By inhibiting the enzyme, acetylcholine accumulates, leading to continuous nerve stimulation, which results in paralysis and death of the louse [1.5.2, 1.5.3]. Mammals have enzymes (carboxylesterases) that can break down malathion more efficiently than insects, which accounts for its relatively low toxicity in humans when used as directed [1.5.3].
How to Correctly Apply Malathion Lotion
Proper application is crucial for both efficacy and safety. A critical safety precaution is that malathion lotion is flammable due to its alcohol base [1.11.1]. During and after application, while the hair is still wet, all sources of heat and open flames (e.g., hair dryers, curling irons, cigarettes) must be avoided [1.11.2].
- Apply to Dry Hair: The lotion should be applied to dry hair in an amount just sufficient to thoroughly wet the hair and scalp [1.3.2]. Special attention should be paid to the back of the neck and the area behind the ears [1.3.1].
- Air Dry: Allow the hair to dry naturally. Do not cover it [1.3.2].
- Leave On for 8-12 Hours: The medication must remain on the hair and scalp for 8 to 12 hours to be effective [1.13.1]. Applying it before bedtime is a common practice [1.3.4].
- Wash Hair: After the 8-12 hour period, wash the hair with a non-medicated shampoo. Rinse thoroughly over a sink rather than in a shower or bath to limit the lotion's contact with the rest of the body [1.3.1].
- Comb for Nits: After washing, use a fine-toothed nit comb to remove the dead lice and eggs from the hair shafts [1.3.1].
- Follow-Up: A second treatment may be necessary 7 to 9 days later if live lice are still observed. This addresses any nits that may have survived and hatched after the initial treatment [1.2.1].
After treatment, it's important to wash all recently used clothing, bedding, and towels in hot water and dry on high heat to prevent reinfestation [1.14.3]. Combs and brushes should also be soaked in hot water [1.14.3].
Off-Label Use: Scabies
While its primary approved use is for head lice, malathion has been used off-label to treat scabies, another parasitic skin infestation caused by the mite Sarcoptes scabiei [1.6.4]. Guidelines in some countries outside the U.S. recommend it as a second-line treatment if others, like permethrin cream, are ineffective [1.6.2]. However, the CDC notes that data on its efficacy for scabies is limited, and it is not FDA-approved for this purpose [1.6.1]. The commercially available lotion in the U.S. contains a high percentage of isopropyl alcohol, which can cause significant burning and irritation on the already excoriated skin typical of a scabies infestation [1.6.4].
Comparison with Other Lice Treatments
Malathion is often compared to other common pediculicides. It's important to understand the differences in application, efficacy, and potential for resistance.
| Feature | Malathion 0.5% | Permethrin 1% | Ivermectin (Oral/Topical) |
|---|---|---|---|
| Mechanism | Acetylcholinesterase inhibitor; neurotoxic to lice [1.5.4]. | Disrupts sodium channels in lice nerve cell membranes [1.7.4]. | Binds to chloride ion channels, causing paralysis and death of lice [1.8.1]. |
| Ovicidal (Kills Eggs) | Yes, considered ovicidal [1.7.4]. | No, not fully ovicidal; requires a second treatment [1.7.4]. | Topical ivermectin is ovicidal; oral is not [1.7.4, 1.8.3]. |
| Application Time | 8–12 hours [1.13.1]. | 10 minutes [1.7.2]. | 10 minutes (topical) or taken orally [1.8.1]. |
| Prescription Status | Prescription only [1.7.1]. | Over-the-counter (Nix) and prescription [1.7.1]. | Prescription only [1.8.1]. |
| Efficacy | High efficacy, often used for resistant lice. May be more effective than permethrin [1.7.2]. | Effective, but resistance is increasingly reported [1.7.4]. | Oral ivermectin was found to be more effective than malathion for difficult-to-treat cases [1.8.1]. |
Safety Profile and Side Effects
When used as directed, malathion is generally safe. The most common side effects are localized skin irritation, such as stinging or burning sensations upon application, and a temporary increase in dandruff [1.4.1, 1.4.3]. If severe skin irritation or an allergic reaction (rash, hives, trouble breathing) occurs, the medication should be washed off immediately and medical attention sought [1.4.1, 1.14.2].
Key safety warnings include:
- Flammability: Avoid all heat sources and flames while hair is wet with the lotion [1.11.1].
- Age Restriction: Not for use on children under 6 years old unless directed by a doctor, and contraindicated in neonates and infants [1.4.4, 1.12.1].
- Pregnancy and Breastfeeding: It is rated as Pregnancy Category B, meaning animal studies showed no risk, but human studies are limited. It should be used during pregnancy or by nursing mothers only if clearly needed and after consulting a physician [1.12.1, 1.12.2].
- Accidental Ingestion: Swallowing malathion can be toxic. Seek emergency medical attention immediately if this occurs [1.3.2].
Conclusion
Malathion lotion is a highly effective prescription medication primarily used for the treatment of head lice and their eggs [1.2.1]. Its neurotoxic action against lice makes it a crucial second-line defense, especially in cases of resistance to common over-the-counter products [1.7.4]. While it has some off-label use for scabies, this is not its approved purpose [1.6.1]. Due to its potency and flammable nature, strict adherence to application instructions and safety precautions is essential for achieving successful eradication of lice while minimizing risks [1.11.1, 1.3.1].
For more information on head lice, you can visit the CDC's Head Lice Information for Schools.
